Optically examining a surface with specular or diffuse reflection for defects during, or immediately after, a manufacturing process has included many different techniques. For example, U.S. Pat. No. 2,867,149 (issued to C. T. Goddard on Jan. 6, 1959) describes a first technique which uses a grating of fine wires, or ruled lines, positioned at an angle to a surface to be measured or examined. Parallel rays of light are then projected through the grating to impinge the surface at an acute angle. When viewed from directly above the surface, the shadows of the grating elements are straight for flat surfaces and non-straight for any deviation in height of the surface. Such technique might be termed a "Zebra" test because of the pattern of light and dark areas generated on the surface. Such technique is capable of seeing surface features of a predetermined minimal size depending on the period (spacing) of the lines of the grating.
A second technique is the well-known Ronchi test, wherein light from a light source is projected through a grating onto a curved reflective surface (e.g., a mirror) under test. The grating comprises alternating opaque and non-opaque parallel areas, and is imaged on itself. An observer images the optical curved surface under test through the grating. Therefore, in a normal Ronchi test, there is literally one grating, and both the light source and the viewing means or eye forming the observer are closely spaced and look through the single grating. In this regard, see, for example, a modified Ronchi test arrangement for measuring a flat surface described by R. W. Harrison in the IBM Technical Disclosure Bulletin, Vol. 12, No. 10, March 1970 at page 1643. In the Harrison arrangement, two lenses are needed to make up for the loss of the curved surface used in the classical Ronchi test.
U.S. Pat. No. 4,810,895 (issued to O. Kafri et al. on Mar. 7, 1989) discloses a third technique for optically examining the surface of an object using Moire ray deflection mapping. With this Moire ray deflection mapping arrangement, light reflected from the surface of an object to be measured is collimated and directed through a first and a second closely spaced grating. The gratings are located at a preselected angular orientation with respect to each other to produce a Moire fringe pattern that provides an indication of the properties of the examined surface. In Moire techniques, the detector (observer or camera) is generally located immediately behind the second grating, or at the image of the first grating on the second grating. For other Moire grating arrangements see, for example, U.S. Pat. No. 3,166,624 (issued to L. O. Vargady on Jan. 19, 1965); and U.S. Pat. Nos. 3,572,924 and 3,604,813 (issued to G. H. Te Kronnie et al. on Mar. 30, 1971 and Sept. 14, 1971, respectively).
Moire techniques are also used to detect a distance or displacement of a surface. Such Moire distance measuring techniques can be used for surface contour measurements or for positioning the surface of an object relative to another object or surface. For example, U.S. Pat. No. 4,733,605 (issued to Livnat et al. on Feb. 2, 1988) discloses a Moire distance measurement method and apparatus useful for the non-contact measurement of small displacements of a specularly reflective surface with a high degree of accuracy. The technique described by Livnat et al. involves projecting a collimated beam of light through a first grating onto the specularly reflective surface. The light reflected by the surface is modulated by a second grating rotated at an angle .THETA. with respect to the first grating to form a Moire pattern which can be used to determine distance. A displacement of the surface causes the Moire pattern to shift, and this shift in the Moire pattern is detected and used to measure the distance moved by the surface.
In general, Moire distance measuring techniques usually require a distance measurement obtained by counting the number of times a measured intensity in a Moire pattern behind a viewing grating goes through a periodic change. This gives the distance in contour intervals from an initial position, where a contour interval is a change of detected intensity at a point in the Moire pattern which goes through a full period (i.e., bright to dark to bright). The information about the position of the surface being measured is lost, however, if the count of the number of contour intervals is lost or wrong.
It is desirable to have a Moire distance measurement technique which does not require the counting of contour intervals to make a Moire distance measurement.